1. Field of the Invention
The invention relates to polycrystalline silicon, a method for producing it and a device for producing it.
2. The Prior Art
High-purity semiconductor material is needed for the production of solar cells or electronic components, for example, memory elements or microprocessors. The dopants introduced in a controlled manner are the sole impurities which should be present in such a material. An attempt is therefore made to minimize the concentrations of harmful impurities. It is frequently observed that even semiconductor material produced under high-purity conditions becomes contaminated again in the course of the further processing to produce the final products. Thus, expensive cleaning steps are repeatedly necessary to reacquire the original purity. Foreign metal atoms which are built into the crystal lattice of the semiconductor material disturb the charge distribution and may reduce the functioning of the subsequent component or result in its failure. Consequently, contamination of the semiconductor material by metallic impurities should be avoided. This applies to silicon which is the semiconductor material most frequently used in the electronics industry.
High-purity silicon is obtained, for example, by thermal decomposition of readily volatile silicon compounds. These compounds, such as trichlorosilane, can therefore be purified easily by distillation methods. Under these circumstances, polycrystalline silicon is produced in the form of ingots having typical diameters of 70 to 300 mm and lengths of 50 to 2500 mm. A large proportion of the ingots is used to produce crucible-drawn monocrystals, strips and foils or to produce polycrystalline solar-cell raw material. Since these products are produced from high-purity molten silicon, it is necessary to melt solid silicon in crucibles. In order to make this operation as efficient as possible, large-volume, solid pieces of silicon, such as, the polycrystalline ingots mentioned, have to be comminuted before melting. This is normally always associated with a superficial contamination of the semiconductor material. This is because the comminution is carried out with metallic crushing tools, such as jaw crushers or roll-type crushers, hammers or chisels.
During the comminution, care has to be taken to ensure that the surfaces of the fragments are not contaminated with impurities. Contamination by metal atoms is to be regarded as critical since these may alter the electrical properties of the semiconductor material in a harmful way. If the semiconductor material to be comminuted is comminuted with mechanical tools, such as steel crushers, the fragments have to be subjected to a surface cleaning before melting.
Mechanically machined polycrystalline silicon or polycrystalline silicon granules which have been produced from mechanically machined products can be used as silicon starting material. This starting material is for the production of monocrystalline silicon. However, it is necessary to reduce the concentration of iron atoms and/or chromium atoms which are present on the surface of the mechanically machined polycrystalline silicon.
Mechanically machined particles of polycrystalline silicon can be used as starting material for monocrystalline silicon. In this case, the surface of the mechanically machined polycrystalline silicon is etched with a mixture of nitric acid and hydrofluoric acid. This process is widely used, but it cannot adequately reduce the concentration of iron atoms and/or chromium atoms on the surface of the polysilicon.
DE-A1 195 29 518 discloses that polycrystalline silicon is first cleaned with a mixture of aqua regia (a mixture of hydrochloric acid and nitric acid) and is furthermore cleaned with hydrofluoric acid. This process, however, achieves only a mean iron value of 73.32.times.10.sup.-11 g/cm.sup.2.
JP 051-54466 describes a cleaning process in which hydrofluoric acid and nitric acid are used. A mean iron value of 23.31.times.10.sup.-11 g/cm.sup.2 was achieved.